1. Field of the Invention
The present invention relates to materials for catalytically oxidizing hydrocarbons; more particularly, to so-called “three-way” materials for catalytically oxidizing hydrocarbons and carbon monoxide, and reducing nitrogen oxides; and most particularly, to three-way tin-oxide based catalytic systems capable of withstanding temperatures of exhaust gases from internal combustion engines.
2. Discussion of the Related Art
Catalysts based on tin oxide (SnO2) and precious metals are well known in the art of catalysis, especially for low-temperature oxidation of carbon monoxide (CO) to carbon dioxide (CO2) in the absence of SO2 and in the presence of only trace amounts of water vapor. See, for example, U.S. Pat. Nos. 4,829,035; 4,855,274; and 4,912,082, the relevant disclosures of which are herein incorporated by reference, which disclose the manufacture and use of such catalysts for maintaining CO2 levels in a CO2 laser, wherein the CO2 gas undergoes progressive degradation to CO during operation of the laser. Such catalysis proceeds at temperatures between about 23° C. and 100° C., the normal operating temperature range of a CO2 laser.
It is further known in the art of precious metal tin oxide catalysts to include one or more “promoters” in a tin oxide-coated substrate. Such promoters can permit large reductions in the amount of precious metal required for desired catalytic activity. For example, U.S. Pat. No. 6,132,694, herein incorporated by reference and referred to as '694, discloses that when no promoter is employed, the catalyst may comprise about 85% tin oxide and up to 15% platinum. When a promoter is employed at a level of about 3 atom percent to tin metal, especially good catalysis is observed at platinum loading of only 1–2% relative to the tin oxide. The promoters disclosed and claimed are all oxides of transition metals such as iron, manganese, copper, cobalt, and nickel.
Precious metal catalysts are widely known for use in catalytically modifying exhaust gases from internal combustion engines, especially in vehicular applications. So-called “three-way” catalysts are able to oxidize CO to CO2, to oxidize hydrocarbons to CO2 and H2O, and reduce nitrogen oxides to N2. Tin oxide is especially attractive as a component of a three-way exhaust catalysis system because of a high inherent oxygen storage capacity. A serious problem exists, however, in applying prior art tin-oxide based catalytic systems to automotive uses. At the relatively high temperatures dictated by engine exhaust, generally between about 500° C. and 1000° C., the structure of tin oxide is seriously affected, as measured by the Brunauer/Emmett/Teller (BET) surface area. The BET surface area is important because it has a major role in controlling the dispersion of the catalytically-active precious or “noble” metal component. Having a high precious-metal dispersion ensures a high level of catalytic activity. As the tin oxide structure loses surface area, the number of catalytic sites decreases due to precious metal sintering, and catalytic activity is diminished. Consequently, tin oxide in any form has not been commercially adapted heretofore in automotive catalyst systems.
What is needed is a means for stabilizing tin oxide structure at high temperatures to permit use of tin-oxide based catalytic systems on internal combustion engine exhaust gases.